You should be able to make a raw salad just like with kale--massaging and such.
I think some kind of wrap would be nice in summer--blanch the leaves and roll around a couscous or grain salad.
They would also be great in a noodle salad--again blanch so they stay bright green and cut into strips.

I found my favorite new thing on the Not Recipes app. Veggie summer rolls with Tahini Miso Dipping Sauce. I've done a veggie version, but I am a big fan of protein, so I've been making it with sliced turkey, sliced beef or roasted shrimp. The sky is the limit.

You par boil the greens with the stem on. Let them cool and remove the stem. Each half makes one roll.

Oops..didn't notice your request for vegetarian. These originally were meant to be vegetables only. I added the protein because they are a main course for me. Tofu would be a good addition too. Either marinated first or not.

Here in Oregon, they are grown all year, so not out of season. They do better in fall and winter, but are often planted in Spring for a Summer harvest. Our apartment community garden is full of collards which we just harvested before this heatwave hit.

We've had success adding them to anything where you would use other greens. my favorites are in soup or pasta. We love it in pho and we usually add twice as much as the recipes describes. It just disappears in the dish.

Collards are super delicious, when substituted for spinach in Spanikopita, and served it's delicious served room temp with a salad. I always blanch my collards in nice salty water for about 2 minutes, squeeze out the water and use in place of spinach.

The traditional accompaniment to southern style collard greens (long simmered with a ham hock or bacon or some form of pig) is, as I'm sure you know, corn bread - but since it's corn season now, you could add some fresh local corn into the batter. Or serve a corn pudding with it - my recipe (on this site) uses fresh corn as well as cornmeal, so in the vein.

I get organic vegetables delivered to my home weekly. I've been experimenting this summer with adding greens to various recipes for extra flavor and nutrition. One of my family's favorite adapted recipes for seasonal vegetables lately has been fried rice. I like to chop the greens really small and saute them lightly before adding to the rest of the dish. Delicious!

Collard greens are pretty much available all year around, and while the local ones that are showing up now are young, this is a vegetable that is generally not tender enough to eat raw. Unless of course, you are a rabbit. ;)
As a summer side dish to "go with" my favorite braised collards, I like Maque Choux. I have a recipe for that in my collection. And cole slaw goes with everything in the summer.

I agree with ChefJune on raw collards. They are down right nasty. I stem them, layer a few leaves and roll those into tight cigars. With a sharp knife i crosscut the cigars into thin filaments. They are now ready to be sauteed in a lipid, or tossed in a soup.